Flexography is a unique printing process, developed primarily for printing packaging and other industrial materials. Packaging materials typically are supplied and processed in roll form, hence flexographic printing systems have developed using various configurations of rolls to feed these materials into the system. The materials on such rolls are often referred to in the art as a web or substrate. U.S. Pat. No. 4,878,427, the specification of which is incorporated herein by reference, describes various devices and mechanisms utilized in such systems. For example, one such system described therein is the simplest and most common form of the flexographic printing systems, consisting of four basic parts: doctor roll, anilox roll, plate cylinder, and impression cylinder. The mechanisms of this system are further described in this specification.
The ink used in flexographic systems has traditionally been thin, highly fluid, and rapid drying. The type of ink used on a particular job, however, may possess different properties. For example, when newer inks are requested or required for printing, they often contain greater pigmentation, and therefore have greater viscosity. The flexographic printing system does allow for more viscous or paste-type inks that are formulated from resins and may be either solvent or water reducible, but their use can affect the performance of the system.
Historically, the inking system of a flexographic printing group was often configured in the prior art using several rolls. As shown in U.S. Pat. No. 4,878,427, for example, these rolls were a doctor roll (sometimes called a rubber roll), an anilox roll, a printing cylinder, and an impression cylinder. The doctor roll, generally made of natural or synthetic rubber, is rotated through an ink reservoir and coated with ink. The doctor roll is configured to rotate against the anilox roll. The rotation and contact between the anilox and doctor rolls acts to transfer the ink from the doctor roll to the anilox roll. The anilox roll is usually made of metal or ceramic coating and is covered with anywhere from 80 to over 1200 tiny cells per lineal inch, called anilox cells. Ink is delivered into these cells by the considerable pressure created at the point of contact between the doctor roll and the anilox roll. The pressure created by the interaction of these rolls is important to meter the amount of ink delivered into the cells and ultimately to the printing cylinder. In this configuration, the pressure also eliminates excess ink from the surface of the anilox roll, leaving the ink primarily in the cells.
The printing cylinder sits between the anilox roll and the impression cylinder. The exterior of the printing cylinder is wrapped with the printing plate, which is often adhered to the cylinder using double-sided adhesive tape and holds the template of the design desired to be printed. The anilox roll rotates against the printing cylinder, coming into contact with the printing plate. This action causes the anilox roll to supply the desired amount of ink to the printing plate. Regulating this supply of ink is of particular importance. The web or substrate is fed into the system at the point where the printing plate contacts the impression cylinder. The ink from the printing plate is impressed onto the substrate as it rolls around the impression cylinder, which serves as a support. The contact pressure between the anilox roll and the printing cylinder is generally set as light as possible such that the material to be printed on is not over inked and the resulting image blurred. It follows that the metering of ink delivery into the anilox cells is very important to producing the desired resulting image. If too much ink permeates the cells, the plate is over-inked; too little ink will cause the plate to be too dry and no acceptable image will be made on the substrate, a situation known as ink starvation.
There have been variations on the basic flexographic printing system in the prior art. One such variation adds a “doctor blade” to the anilox roll just beyond the ink metering location where the doctor roll rotates against the anilox roll. Its purpose is to shave the surface of the anilox roll to remove surface ink and insure a more controlled delivery of ink to the printing plate. The doctor blade is often adjustable to allow more or less contact with the anilox roll and to compensate for variations in the roll's diameter. In the traditional four roll flexographic system, the doctor blade acts as an additional ink metering device in conjunction with the pressure between the doctor roll and the anilox roll. The pressure forces ink into the anilox cells and the doctor blade increases the accuracy of the ink delivery system to the printing plate.
Another variation eliminates the doctor roll altogether. As described in the '427 patent, an ink applicator pumps a heavy flow of ink to the anilox roll from a remote tank. A doctor blade is positioned just beyond the applicator. An ink reservoir pan serves as a catch basin below the anilox roll for funneling ink back to the remote ink tank.
Both of these systems present significant disadvantages. Yet another variation that has been tried utilizes an ink chamber that is set up beneath and in conjunction with the doctor blade. This system has a closed chamber that is filled with ink through an ink tube and pumped in from a remote ink tank. The pressure in the chamber may be adjusted to keep the anilox cells filled to the desired volume by adding or removing ink using the tube system.
As technology has evolved, ultra violet, solvent based, and stocked water based inks have become more popular. These inks have a higher viscosity than the standard water based ink. This poses difficulties for the three cylinder flexographic system because the higher viscosity of the inks makes it more difficult to fill the anilox cells to the desired volume. There is no pressure means in such a system which would force the thicker, more viscous inks to fill the anilox cells to the desired volume. As a result, less ink is impressed onto the printing plate than is needed and ink starvation often occurs. When the anilox roll is rotated through the ink applicator at certain speeds, the transfer of the ink from the application tray to the anilox cells becomes completely erratic. Moreover, the viscosity of the ink presents a great disadvantage to the system that employs a pump system. The tubes that feed the chamber from the remote ink tank become clogged with more viscous inks and require constant cleaning. Cleaning in this system is very difficult and required maintenance in excess over the cost of using it. The present invention eliminates this problem and the problem of ink starvation found in the prior art by providing an economic alternative to prior art systems.